Exhaust Flow and Aero

[hotrod]

The largest, and most obvious issue with suction as a way to control boundary layer is that the holes traditionally used for that are likely to plug quite rapidly in the salt environment.

...

I personally have no issue with cleaning the car after each pass, but it would be dificult to get small holes to clean in a timely maner, and large holes would be difficult to supply with enough flow to generate the necessary boundry layer reduction.

If you have a suggestion as to how to avoid that issue, or clean the systems out faster I would love to hear it.

Good point, perforated panels and narrow slots would likely not be desirable in either the salt flats or the dry lake environments.

As I mentioned in my post, the NACA duct by its very design strips off the boundary layer. As the air flows down the leading edge ramp and spills over the curved inlet edges toward the rear of the duct, the air closest to the surface (the boundary layer) is the first to go. The free stream flow then would be inclined to re-attach to the surface just behind the NACA duct, especially if you had some mechanical suction being applied to the duct with an internal fan to assist the flow.

One interesting thing to note is that the seam between panels could also be constructively used for such an air bleed system. Many modern cars place their outdoor thermometer sensor in the door jam of the front edge of the drivers side door. There is air flow into that seam area. If you applied suction into a body panel seam on a streamliner only a 1/16" inch wide or so just in front of the area you are trying to preserve attached flow, I suspect you could strip off a good portion of the thick boundary layer at that point (depending on the width of the gap, applied suction level and shaping of the panel immediately behind the air bleed gap).

One advantage of using a panel seam, is cleaning would be trivial, pull the panel off in the pits and clean the now exposes edges of the seam.

Those are just two ideas that come to mind. The only way to know if it works is to do some before and after tuft testing to see if the surface turbulence behind the air bleed slot is reduced. There may be a critical value of suction needed, too little might do you little to no good, and too much might also cause problems like deforming the panel.

Fun to play with as a mind puzzle but only testing will tell you if it is a workable idea. If you have to pull cooling air someplace anyway, it only makes sense to try to locate the inlets in places that would help you maintain attached flow.

Larry

[superford317]

hotrod, i wish to apologize to you, i have pulled 5 straight 19 hour days and when i made my last post it was without very much thought or consideration.
when i went to my room last night, i thought about it for a long while and it really stuck in my mind all day.
i fell asleep 5 times today reviewing some research, i really need some rest.
for obvious driver safety, the exhaust heated panels and the water flowing along the body can be used on the rear half of the vehicle and electric heaters can can be used at the front.
exhaust augmenters, cooling ducting and venturies can be used to pull a vacume on the trouble areas of the body to help the aero.

[manta22]

"...electric heaters can can be used at the front"

Calculate the electrical power required to heat a large surface in a high velocity airflow--beaucoup watts!

Regards, Neil  Tucson, AZ

[superford317]

a large area would not have to be heated, a band around the body as close to the front as possible would give a measurable benefit as the radiated heat would be carried along the boundary layer.
on a streamlined body think of the boundary layer as a cushion of air sitting under a blanket.

[k.h.]

I've resisted posting again, but the temptation for searching got the better of me.

http://mb-soft.com/public/lowdrag.html

"In 1966, Northrop and the U.S. Air Force ... built two of the largest X-planes ever flown: the X-21As. The experimental twin-jets started life as weather-reconaissance Douglas WB-66 jets, electronic warfare versions of which saw service over Vietnam. Under a distinctive humped back, each X-21 sported a swept laminar flow control wing lined with thousands of spanwise razor-thin slits that were in turn perforated with over 815,000 minuscule holes, each of which sucked away turbulent air into a vast internal network of nearly 68,000 ducts, all leading to a pair of high-pressure pumps under the wings. The B-66's main engines were moved from their under-wing pylons to aft shoulder mounts like those on a typical business jet.

The X-21s were meant to prove not only that active laminar flow was achievable but that such a system could be manufactured, maintained, and operated in an everyday environment. "The X-21As proved conclusively that...[laminar flow control] is both effective and viable," experimental-aircraft authority Jay Miller writes in his book The X-Planes. "However, they also demonstrated that LFC incurred certain maintenance penalties that were not easily overcome...[and] that production technology for manufacturing LFC surfaces and related components was...prohibitively expensive for all but experimental aircraft."

The X-21A program had demonstrated that active laminar flow could be achieved using a hand-built wing that required constant maintenance--much of it devoted to keeping the pinholes from clogging with dust, dirt, and bugs--and enough power on board to run the hungry pumps. Active laminar flow control seemed to be a laboratory oddity with no hope of practical application. Unfortunately, that may be nearly as true today as it was in 1966.

The size and shape of the pinholes--and tuning the exact amount of suction applied through them--are the keys to the success or failure of any active laminar flow control system. In the 1940s and '50s, the trick was drilling the holes small enough or finding a porous wing material strong enough. In the '60s and '70s, the holes got smaller and more precise, but the problem became one of keeping them from clogging with dust and dirt."
Air & Space/Smithsonian Magazine, JUN/JUL 1995.

[superford317]

what i have been proposing is only use the vacume on the most trouble areas, if its only 24 sq/in on each side of the body at the worst areas, it is a start to better aero package.

[k.h.]

It's worth trying.

[manta22]

superford:

"...the radiated heat would be carried along the boundary layer..."

Not "radiated" heat-- I think you mean "conducted" heat.

BTW--Did you do that electrical power calculation?

Regards, Neil  Tucson, AZ

[John Burk]

I sort of doubt the air wizzing by would pick up much heat . The underhood air in a production car at cruising speed after passing through condenser and radiator is only about 2 degrees warmer than outside . The guy who designed the 1970 Firebird fairings and hood scoop told me that . The air conditoning department got worried about the intake air bypassing the condenser and didn't realize .

[Nexxussian]

Sure, it's worth a try (boundary layer control), and if you could work it where a panel gap was where you are drawing the air, it would definetly expedite cleanout. 

Of course, if it was a production based car, it could get the nickname "mister stich" if you tried that.

Not that it would be leagal in a production class.

Hotrod, how far to each side would you expect the boundary layer would a NACA duct strip away the boundary layer?

Sparky, I thought of that, but my luck it would fail the system, catastrophically. 

I'm sure our freindly officials would take a dim view of a "cleaning" procedure that could result in spreading pieces of the body around in a hurry, but maybe I'm wrong, someone could test that, on their car; when I am far, far away. 

I could try driving water backwards through the system, but visions of the car looking like a sprinkler aren't the best, not to mention the soft spot that would leave in the salt.

The officials wouldn't like that either. 

[hotrod]

Hotrod, how far to each side would you expect the boundary layer would a NACA duct strip away the boundary layer?

That is a very good question!

It would effectively remove the boundary layer buildup for its entire width, and I suspect an area about 1/2 its width on each side of the scoop, but that is only a scientific wild "ass" guess. In the area behind the scoop you would have a very thin boundary layer and the thicker boundary layer areas on both sides "should" be influenced by that faster laminar flow nearby. What I am visualizing is that the nearby boundary layer would be gradually accelerated and in effect blown away by the laminar flow behind the NACA scoop. Sort of like what would happen if you have a fast flowing stream of water injected into a slower moving body of water. The slow moving water is accelerated and the fast moving water slows down as the two exchange momentum.

That would be something to be tested by someone in the A2 tunnel, or with tuft testing.
But that would be my expectation for the results of the testing.

Larry

[Robin UK]

The F1 guys have been using blown diffusers to great effect this year - so much so that the FIA are trying to put severe controls on them and causing a fair old row. Clever engine mapping to cut cylinders (which makes the engines sound horrible) means they can still blow high pressure exhaust gasses through the diffuser to increase grip and overall aero effieciency even when the driver is off the throttle in lower speed corners. You'll read this refered to as cold blown vs hot blown. Renault Lotus go as far as exiting the exhausts forward to blow gasses into the front of the sidepods and then out to the diffuser to maximise the hot blown effect. These guys sepnd millions just to get a fraction of a second improvement per lap (not saying that's a good thing but it's the way it is) so you can safely say that it works for them. Worth more investigation I'd say. Here's a link to start - there are lot's more.

http://en.espnf1.com/f1/motorsport/story/21825.html

Robin

[floydjer]

Is the "Propster" alive  and well  or is that a pusher Canard  plane

He`s alive, But as for "Well"...................

[SPARKY]

 

[superford317]

Sorry for the long post but it is somewhat of a complicated subject so I wanted to explain it as simply as I could.
Most of this post will pertain to coup class or full bodied vehicles, as streamliners have a super efficient body design and handle the airflow around them and the wake behind them quiet well with little pressure drag.
Several posts ago I mentioned vehicle surface shape and texture having an influence on drag and “maj” from Australia later asked what I thought about the “sharkskin” and its effect on boundary layers, its not an easy answer and like many things will depend on the shape, size and speed of the vehicle it is applied to.
Shark scales are called placoid and have tiny ridges on them that are parallel with the direction the shark swims in, and differ from one shark species to the next, the faster the particular shark species could swim determined the size of the riblets.
The sharkskin, riblets, chevrons, bumps or V’s, I prefer to call them riblets, whatever you wish to refer to them as, do there work in turbulent boundary layers. The size of the riblets will depend on the thickness of  the boundary layer at the vehicles surface and the speed of the vehicle through the air.
In LSR racing, pressure drag is more detrimental than friction drag.
Air has mass and it is displaced and moves around the vehicle as the vehicle passes through it, the force produced and the amount of air displaced is determined by the vehicle shape, speed and air density. Air moving past the vehicle sticks to the surface slows down and forms a layer, the air layer adjacent to the vehicle surface remains attached to the body surface, the air above that layer slows a little less and so on until there are several slower than free stream air velocity layers and this is called the boundary layer. How thick the boundary layer is depends on the vehicle speed, shape and air density. As a vehicle accelerates it disturbs more air than it does at slower speeds. Air moving in the boundary layer causes friction drag on the vehicle. Movement between the air layers remove energy and convert it to heat, lost energy from the boundary layer will cause a transition to turbulent flow and can take place even over a smooth vehicle surface.
The boundary layer can be laminar or turbulent. Laminar layers have less vehicle skin friction than a turbulent layer and drag will be less. If the boundary layer is laminar, changes in the speed of the air in the layer is gradual as it moves away from the vehicles surface. If the boundary layer is turbulent the air speed is chaotic and vortices form inside the boundary layer and will separate away from the vehicles surface because it has lost energy and is acted up on by the free stream air pressure.
Turbulent air has a quick change in speed and pressure and forms vortices that react with each other, increasing friction drag at the vehicle surface.
As the boundary layer moves from the front to the back of the vehicle it looses energy and will thicken and become turbulent or an abrupt change in the body shape or vehicle surface texture can cause the flow to totally separate from the vehicle and form a turbulent area behind the vehicle called a wake. The wake will cause a lower air pressure immediately behind the vehicle than there is at the front of the vehicle and that is what creates the pressure drag, think of it as a large suction pulling on the back of the vehicle and slowing its acceleration rate. Vortices are formed in the wake and will continue far behind the vehicle until the energy is overcame by the air viscosity and turned into heat. That is why I stated in many of my earlier posts that it is best to dump all of the air and heat that you can into the wake behind the vehicle, to help reduce the low pressure area and lessen the pressure drag on the vehicle.